1. Field of the Invention
The present invention relates generally to fluid flow systems. In one of its aspects, the invention relates to the control of fluid flow in a wind tunnel system, and the elimination of extraneous noise and pressure fluctuation within a test section of the wind tunnel system.
2. Description of Related Art
Generally, the purpose of a wind tunnel is to measure the effect of the passage of a high velocity fluid, in this case air, over a body under controlled conditions. Such body may be an airplane, a building structure or an automobile.
In a wind tunnel, the prime consideration is to be able to control the velocity and the uniformity of the air flow. One very impractical method of doing this is to have a very long straight wind tunnel with the right combination of cross-sectional area and input wind generation power. This is impractical because each end of such a wind tunnel must be open to the atmosphere. Therefore, the common practice is to make the wind tunnel a loop so that no make up air is needed, debris can be prevented from entering the tunnel, energy is conserved, and other factors, such as air temperature, can be controlled.
Additionally the wind tunnel must simulate as closely as possible the wind environment that is present around the vehicle when it is moving in its natural environment. To accomplish this, wind tunnels with straight solid walls are made with very large test sections so that the vehicle under test blocks only about 5% of the cross section of the wind tunnel throat. In an open jet wind tunnel (or semi-open jet wind tunnels which have a floor surface) the tunnel can be built much smaller due to the fact that the test vehicle can block as much as 15% of the throat area without significant distortion to the air flow field around the vehicle. However, the open jet tunnel design introduces the distinct possibility that a hydro-acoustic resonance will be set up within the tunnel circuit that causes fluctuations of the velocity of the air in the test section. These fluctuations occur as various acoustic feedback paths excite the fluid field inside of the wind tunnel. The resulting unsteady flow field will no longer support appropriate aerodynamic or aero/acoustic wind tunnel testing. This is due to the fact that the aerodynamic forces vary in proportion to the velocity squared in the testing, and the noise amplitudes rise with the fifth sixth order of wind velocity. So the fluctuations must be reduced or eliminated to achieve satisfactory test conditions.
The solution of this resonance has been approached from a variety of different directions. Different dimensions of the open jet wind tunnel test section and various designs of collector “flaps” have been utilized with varying degrees of success. Some of these solutions resulted in test sections that were too short for the vehicles being examined causing distortions in aerodynamic data. Other solutions isolated the resonance phenomenon to specific speed ranges, but did not eliminate it.
For the purposes of aerodynamic evaluation of solid bodies, it would be advantageous to provide a wind tunnel assembly that minimizes unsteady pressure fluctuations, speed variations with time, pressure field distortions and the noise generated and attributable to the wind tunnel itself, under the entire range of operating speeds.